Support openmw animation overrides

[magicaldave]

Partial resolution for MO2 #1609
Simply adds support for a base directory named animations. This is in accordance with the openmw convention of allowing per-group animation overrides.

One example of such a mod would be:
Simply Sheathing
And also:
Weapon Sheathing

Note that Scripts directories are not supported as the root directory of such a mod will always contain a valid plugin.

Unfortunately this is not a complete solution, but since users can now provide arbitrary directories which contain arbitrary file types with openmw, there is not a reliable way to determine if script-related content constitutes a valid data directory other than expecting an omwaddon or omwscripts file to exist in the root.

[AnyOldName3]

This has nothing to do with #1609 as that's just discussing content files. Either way, it's sensible.